1. Field of the Invention
This invention relates to a liquid crystal display, and more particularly to a liquid crystal display and a fabricating method thereof.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) controls light transmissivities of liquid crystal cells in response to video signals. An active matrix LCD provided with a switching device for each liquid crystal cell is suitable for displaying moving images. A thin film transistor (TFT) is used as a switching device in the active matrix LCD.
FIG. 1 is a plan view showing a structure of a conventional LCD, and FIG. 2 is a cross-sectional view of the conventional LCD along A-A′ in FIG. 1.
In FIGS. 1 and 2, the conventional LCD includes a TFT having a gate electrode 13, a gate insulating film 15 and an active layer 17 disposed on a substrate 11. The LCD also includes a source electrode 21 and a drain electrode 23 on the active layer 17, and a pixel electrode 29 that is electrically connected to the drain electrode 23 via a contact hole 27. The TFT applies a data signal from a data line 24 to a pixel electrode 29 during a period in which a scanning pulse is applied to the gate electrode 13. The gate electrode 13 is electrically connected to the gate line 14 while the source electrode 21 is electrically connected to the data line 24. The drain electrode 23 is electrically connected to the pixel electrode 29 via a contact hole 27 formed in a protective layer 25. The pixel electrode may be formed of a conductive material such as indium-tin-oxide (ITO), indium-zinc-oxide (IZO) or indium-tin-zinc-oxide (ITZO). The gate insulating film 15 may be formed of an inorganic insulating material and the source electrode 21, the data line 24, and the active layer 17 may be formed on the gate insulating film 15. The protective layer 25 may be made from an inorganic insulating material or an organic material.
FIGS. 3A to 3E show a process of fabricating the conventional LCD shown in FIG. 1 and in particular shows the TFT portion of the LCD.
In FIG. 3A, aluminum (Al) or a copper (Cu) is deposited on a transparent substrate 11 by a sputtering process, for example, to form a metallic thin film. The metallic thin film is patterned by photolithographic and wet etching processes, thereby forming the gate electrode 13.
In FIG. 3B, a gate insulating film 15 is formed on the transparent substrate 11 to cover the gate electrode 13. The gate insulating film 15 is formed by deposition of an insulation material such as silicon oxide or silicon nitride. An active layer 17 and an ohmic contact layer 19 are sequentially formed on the gate insulating film 15 by a chemical vapor deposition (CVD) process, wherein the active layer 17 is formed from an amorphous silicon or a polycrystalline silicon that is not doped with an impurity. The ohmic contact layer 19 is made from an amorphous silicon or polycrystalline silicon doped with an n-type or p-type impurity at a high concentration. The ohmic contact layer 19 and the active layer 17 are patterned by photolithographic and anisotropic etching processes, thereby exposing the gate insulating film 15. Portions of the active layer 17 and the ohmic contact layer 19 remain only at a portion corresponding to the gate electrode 13.
In FIG. 3C, molybdenum (Mo) or a molybdenum alloy such as molybdenum-tungsten (MoW), molybdenum tantalum (MoTa) or molybdenum-niobium (MoNb), is deposited on the gate insulating film 15 by a CVD or sputtering process to cover the ohmic contact layer 19. The metal or the metal alloy deposited in this manner makes ohmic contact to the ohmic contact layer 19. Then, the metal or the metal alloy is patterned, along with the ohmic contact layer 19, by photolithographic and etching processes, thereby exposing a portion of the active layer directly above the gate electrode 13 and forming the source electrode 21 and the drain electrode 23.
In FIG. 3D, an inorganic insulating material such as silicon nitride or silicon oxide, or an organic insulation material having a small dielectric constant such as an acrylic organic compound, Teflon, BCB (benzocyclobutane), Cytop or PFCB (perfluorocyclobutane), is deposited on the gate insulating layer 15, thereby forming a protective layer 25. Then, a contact hole 27 is formed in the protective layer 25 exposing the drain electrode 23.
In FIG. 3E, a transparent conductive material such as ITO, IZO or ITZO is deposited onto the protective layer 25 and into the contact hole 27, thereby forming the pixel electrode 29. The pixel electrode 29 electrically contacts with the drain electrode 23 via the contact hole 27.
However, in the conventional LCD, since the contact hole must be formed in the protective layer to electrically connect the drain electrode to the pixel electrode, an aperture ratio is reduced and a complicated process is required.